STUDY OF H-BRIDGE & 4-QUADRANT CHOPPER
AIM:
1. To Study the 'Speed control of a DC motor using BJT H-bridge' by using Simulink.
2. To Study the 'Four-Quadrant Chopper DC Drive (DC7) block' by using Simulink.
3. To prepare EV model using DC7 block in Simulink.
KEYWORDS: Chopper, H-bridge, DC motor, IGBT
SOFTWARE: Matlab, Simulink
OBJECTIVE:
1. Study the 'Speed control of a DC motor using BJT H-bridge' and modify the pulse ratio which doesn't shootup the armature current when the motor changes the direction.
2. Study the 'Four-Quadrant Chopper DC Drive (DC7) block' and compare it with H-bridge by taking advantage and disadvantage of both.
3. Prepare EV model using DC7 chopper block and make a result by changing the suitable load torque.
INTRODUCTION:
Study 1:-
The above figure shows the Simulink model of 'Speed control of a DC motor using BJT H-bridge', which consists of DC motor source, Bi-polar junction transistor (BJT), and Diodes. The diodes and transistors are connected in the form of "H" construction. The DC motor has a preset model (5 HP 24V 1750 rpm), it can be changed according to our requirement. It simulates a fan type load (where Load torque is proportional to square of speed). The armature mean voltage can be varied from 0 to 240 V when the duty cycle (specified in the Pulse Generator block) is varied from 0 to 100%. The Bipolar Junction Transistor when used for power switching applications operates as an IGBT. When it is conducting (BJT operating in the saturated region) a forward voltage Vf is developed between collector and emitter. Because of the transistors are connected in H-bridge, the back emf is neglected.
An H-bridge is an electronic circuit that switches the polarity of a voltage applied to a load. These circuits are often used in robotics and other applications to allow DC motors to run forwards or backwards. The H bridge consists of four BJT/Diode pairs (BJT simulated by IGBT models). Two transistors are switched simultaneously: S1 and S4 or S2 and S3.
Case I: Forward Motoring
When S1 and S4 are fired, a positive voltage is applied to the motor which rotates in clock-wise rotation, the torque produced Armature current are positive value thus power is positive and diodes D2-D3 operate as free-wheeling diodes when S1 and S4 are switched off.
Case II: Reverse Motoring
When S2 and S3 are fired, a negative voltage is applied to the motor which rotates in anticlock-wise rotation, the torque produced & Armature current are negative value and diodes D1-D4 operate as free-wheeling diodes when S2 and S3 are switched off.
Based on the default value of DC motor, Armature mean voltage (0 to 240 V), Step time 0.5, duty cycle (specified in the Pulse Generator block is 75%) is varied from 0 to 100%,and forward voltage (1 V) & resistance (0.001 ohms) of four BJT/Diode pairs. The result obtained for Armature current are shown below, which represents sudden raise and sudden fall of current at the beginning and at step time (0.5) respectively.
To avoid the shootup marked in red circle, the duty cycle and period should vary by using trial & error method. The modified parameters are period as 1ms which inturn increasing the frequency (1000 Hz) and reducing the duty cycle to 45% which is optimised. If the forward voltage altered in BJT/Diode pairs, there is less variation occured and the torque & Speed as changed. The modified Armature current as shown in output.
Study 2:-
The above model consists of 3-phase source, AC-DC converter includes rectifier and braking chopper, 4-Quadrant chopper DC motor drive, and Quadrant selector.
A chopper is a static device that converts fixed DC input to a variable DC output voltage directly. Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another. Due to this, current and coltage spikes occurs. To avoid this snubber RC circuit is used. The chopper is widely used in motor control and regenerative braking. A snubber is an electrical device that prevents voltage spikes due to sudden changes in current. These voltage spikes, or transients, can damage the circuit and cause arcing and sparks. One type of electrical snubber is the RC snubber, which is composed of a resistor in parallel with a capacitor. Transients are usually caused by switches in the circuit.
4-Quadrant Chopper:
The above figure consists of Voltage source, 4 pairs of chopper & diode, armature resistance, inductance and seperately excited DC motor.
First-Quadrant: Forward Motoring
The chopper-1 & chopper-4 are switched ON, in which the current flows from Chopper-1 to chopper-4 through inductance (which stores energy) the load current and armature current are same and positive thus the motor rotates in forward direction. If the chopper-1 is switched OFF, the energy stored in the inductance flows to chopper-4 which activates the D2 and forms closed loop.
Second-Quadrant: Forward Regenerative Braking
The chopper-2 is switched ON D4 is activated, the current flows from D4 to Chopper-2 through inductance (which stores energy) which is in opposite direction (negative current) and the voltage is positive thus the motor stops in forward direction. If the chopper-2 is switched OFF, the D4 & D1 are conduct current.
Third-Quadrant: Reverse Motoring
The chopper-2 & chopper-3 are switched ON, in which the current flows from Chopper-3 to chopper-2 through inductance (which stores energy) the load current is negative and armature current and also the voltage is negative which is less than the back emf thus the motor rotates in reverse direction. If the chopper-3 is switched OFF, the energy stored in the inductance flows to chopper-2 which activates the D4 and forms closed loop.
Fourth-Quadrant: Reverse Regenerative Braking
The chopper-4 is switched ON D2 is activated, the current flows from Chopper-4 to D2 through inductance (which stores energy) which is in same direction (positive current) and the voltage is negative thus the motor stops in reverse direction. If the chopper-4 is switched OFF, the D2 & D3 are conduct current.
By comparing the Chopper model with H-bridge, each model as certain working operations and some applications. To control the motor in H-bridge, we need to change the polarity which turns into opposite direction. In Chopper model, we need to interrupt one signal under control of another. Thus it may form some spikes of current and voltage, inorder to avoid this Snubber RC circuit is used but in H-bridge it is not required. The H-bridge simulation is continous while the Chopper is Discrete which take little more time than H-bridge. In H-bridge Regenerative braking is not possible, while in Chopper it is possible. The H-bridge is mainly used for Robotics application, while Chopper is mainly used for motor control and Regenerative braking. In chopper the efficiency is more and maintainance is less than H-bridge.
Study 3:-
The above figure shows the EV model using DC7 block which consists of DC Voltage Source, 4-Quadrant DC chopper, controller, and reference speed includes Slider gain. The subsystem of DC Voltage Source, and Vehicle torque-speed characteristics are shown below. The DC voltage is given as 440 V, chopper frequency is 1000 Hz, Nominal speed is 2000 rpm, and rest of the parameters are constant in the model. The speed reference is given interms of slider gain with the range of (-2000 to 1000) and export to the workspace.
OUTPUT:
Study 1:-
Study 2:-
Study 3:-
Drive_cycle includes the data of 'Time' and 'Speed'
Report from the model:
https://drive.google.com/open?id=1yIfzyTGCEw3n5q_w63dFK-79ghkeVpAM
CONCLUSION:
By comparing the Chopper model with H-bridge, each model as certain working operations and some applications. To control the motor in H-bridge, we need to change the polarity which turns into opposite direction. In Chopper model, we need to interrupt one signal under control of another. Thus it may form some spikes of current and voltage, inorder to avoid this Snubber RC circuit is used but in H-bridge it is not required. The H-bridge simulation is continous while the Chopper is Discrete which take little more time than H-bridge. In H-bridge Regenerative braking is not possible, while in Chopper it is possible. The H-bridge is mainly used for Robotics application, while Chopper is mainly used for motor control and Regenerative braking. In chopper the efficiency is more and maintainance is less than H-bridge.